The Solar System and Orbits

On a clear night the planets shine among the stars, wandering slowly against the fixed patterns of the constellations — which is exactly how the ancients spotted them and named them "planets", from the Greek for "wanderers". Our own corner of the Universe, the Solar System, is a family of eight planets and countless smaller bodies all held in orbit around a single ordinary star, the Sun. What keeps everything circling rather than flying off into space is one familiar force: gravity. This lesson describes the structure of the Solar System, explains how the Sun formed, shows how gravity provides the force that holds objects in orbit, and looks at the different orbits used by satellites.

By the end of this lesson you should be able to describe the structure of the Solar System, explain that the Sun is a star that formed from a nebula, state that gravity provides the force keeping planets, moons and satellites in orbit, describe qualitatively how orbital speed depends on orbital radius, and distinguish between geostationary and low polar satellite orbits.

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The Structure of the Solar System

The Solar System is made up of the Sun at the centre and everything that orbits it. Its main components are:

• The Sun — a star at the centre, providing the light and heat, and by far the most massive object in the Solar System.
• The eight planets — in order of distance from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The four inner planets are small and rocky; the four outer planets are large gas or ice giants.
• Dwarf planets — smaller bodies such as Pluto and Ceres that orbit the Sun but have not cleared their orbital path of other objects.
• Moons (natural satellites) — bodies that orbit a planet rather than the Sun directly; the Earth has one Moon, while Jupiter and Saturn have dozens each.
• Asteroids — lumps of rock, most found in the asteroid belt between Mars and Jupiter.
• Comets — bodies of ice and dust on long, very elongated (highly elliptical) orbits that take them far out beyond the planets and then back close to the Sun.

flowchart TD
  Sun[The Sun: a star at the centre] --> P[Eight planets: Mercury to Neptune]
  Sun --> D[Dwarf planets: e.g. Pluto]
  Sun --> A[Asteroids: mostly in the belt]
  Sun --> C[Comets: long elliptical orbits]
  P --> M[Moons orbit the planets]

The planets and most moons follow nearly circular orbits, while comets follow very elliptical orbits — which is why a comet is only visible for a short time as it swings close to the Sun.

Exam Tip: Learn the planet order Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune (a mnemonic such as "My Very Easy Method Just Speeds Up Naming" helps). Remember moons orbit planets, while planets, dwarf planets, asteroids and comets orbit the Sun.

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The Sun: A Star Formed from a Nebula

The Sun is a star — a huge ball of hot gas, mostly hydrogen, that releases energy by nuclear fusion in its core, where hydrogen nuclei join to form helium. It is a fairly average, middle-aged star; it appears so much bigger and brighter than the other stars only because it is so much closer to us.

The Sun, like all stars, formed from a nebula — an enormous cloud of dust and gas in space. Under the pull of its own gravity, the cloud was slowly pulled together, becoming denser and hotter at its centre. When the centre became hot and dense enough, nuclear fusion began, and the Sun started to shine. The leftover material orbiting the young Sun gradually clumped together to form the planets, moons, asteroids and comets — so the whole Solar System formed from the same original cloud.

Exam Tip: The sequence is nebula → (gravity pulls it together) → star. Say that gravity causes the cloud of dust and gas to collapse, and that the star begins to shine when nuclear fusion starts in its hot, dense core.

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Gravity and Orbits

Left to itself, a moving object travels in a straight line. To make an object travel in a circle, a force must continually pull it towards the centre of the circle. For planets, moons and satellites, that force is gravity — the attractive force between any two masses.

• The Sun's gravity pulls on each planet, keeping it in orbit around the Sun.
• A planet's gravity pulls on its moons (and on artificial satellites), keeping them in orbit around the planet.

Gravity acts as the centripetal force — the inward force needed to keep an object moving in a circle. It constantly changes the direction of the object's velocity (pulling it "inwards" from the straight line it would otherwise follow) so that it curves around in an orbit, without changing its speed in a circular orbit. Because the velocity is always changing direction, an orbiting body is continually accelerating towards the centre, even at a steady speed.

This last point often puzzles students, so it is worth unpicking. In everyday language we say something is "accelerating" only when it speeds up, but in physics acceleration means any change in velocity, and velocity includes direction as well as speed. A planet in a circular orbit travels at a steady speed, yet its direction is changing every instant as it curves around, so its velocity is changing and it is therefore accelerating — always directed towards the centre of its orbit. The force causing that acceleration is gravity. A useful way to picture it is to imagine what would happen if gravity suddenly switched off: the planet would no longer be pulled inwards, and it would fly off in a straight line at a tangent to its orbit, exactly as a ball on a string flies off if the string snaps. Gravity is the "string" that keeps pulling the planet inwards, bending its straight-line motion into a closed orbit. Understanding this is the key to answering the very common question "what keeps a planet/moon/satellite in orbit?" — the answer is always an inward (centripetal) force provided by gravity.

Exam Tip: For any orbit, the answer to "what keeps it in orbit?" is gravity, acting towards the centre. Gravity provides the centripetal force and continually changes the direction of the object's motion, keeping it moving in a circle.

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How Orbital Speed Depends on Radius

For a stable circular orbit, the speed of the orbiting object is set by the radius of the orbit. The rule (qualitative at GCSE) is:

An object in a smaller orbit (closer to the body it orbits) must travel faster; an object in a larger orbit (further away) travels more slowly.